1. Field of Invention
The present invention relates to an image displaying technique, and more particularly to an adaptive feedback control method, suitable for performing an adjustment timely according to a frame content to achieve a backlight color field with a minimum image color difference, thereby alleviating a color break-up (CBU) phenomenon of a field sequential color (FSC) display.
2. Related Art
A conventional liquid crystal display (LCD) utilizes a color filter to achieve full-color effects, but the luminous efficiency thereof is not desirable. Based on a fast-response liquid crystal panel, such as an optically compensated bend (OCB) mode, and a backlight source, such as a high-efficient light-emitting diode (LED), developed in recent years, an LCD with a field sequential color (FSC) mechanism has been achieved. Particularly, the speed for sequentially displaying main color fields of red, blue, and green is higher than a time resolution of a response of human eyes, so that the full-color effects can be achieved without requiring any color filter. Through combining the backlight of LEDs with the liquid crystal panel in the OCB mode, an FSC-LCD is expected to become a color LCD with a high luminous efficiency, low power consumption, and low material cost.
However, generally, the critical problem of a conventional FSC-LCD lies in a color break-up (CBU) problem. The CBU problem is caused by a relative movement between an object in an image and eyes of an observer, that is, during a saccade interval of human eyes, a signal from human eyes to human brains is suppressed due to a saccadic suppression. Referring to FIG. 1, a CBU image simulated with an RGB FSC is shown. In the circled position, the CBU phenomenon can be recognized, and as a result, the definition of the whole image is deteriorated. FIG. 2 is a schematic view of a color image displaying method in a conventional FSC. Referring to FIG. 2, under a circumstance that an observation spot is moved as time elapsed, the CBU phenomenon can be found through a pattern displayed in an FSC color displaying manner. In the FSC color displaying manner, an image is displayed in a time sequence, and a color sequence thereof is “RGB RGB RGB . . . ”, in which R represents a red sub-frame, G represents a green sub-frame, and B represents a blue sub-frame. Taking a white image W10 as an example, when it requires to display a white image, in the white image W10 as seen from the observation spot, a combination of B, B, and G is presented on one edge W11 (on the left of FIG. 2) of the white image W10 and a combination of R, R, and G is presented on the other edge W12 (on the right of FIG. 2), which is the so-called CBU phenomenon.
Considering the FSC applications, U.S. Pat. No. 5,337,068 has disclosed a FSC display system and a method for forming an image, in which a liquid crystal device is used together with backlights in three colors of red, blue, and green. The three backlights emit lights respectively, and then the liquid crystal device simultaneously adjusts the light flux respectively, thereby constituting sub frames in three different colors, and finally, the red, blue, and green sub frames are formed into a color frame. As for the conventional FSC system architecture and the method for forming an image, the CBU phenomenon is rather obvious, which can be easily recognized by the observers.
U.S. Pat. No. 6,570,554 has disclosed an LCD, in which sub color fields of three consecutive frames are regularly converted to solve the CBU problem of the conventional FSC-LCD. When the observer tracks an animation object with his/her eyes at the same speed, an integral result of the three consecutive frames is left on the retina of human eyes without generating the CBU phenomenon. Unfortunately, in this method, when the frequency of the green color field is lower than 50 Hz, the human eyes can perceive a flicker phenomenon, and as a result, the frame quality is deteriorated.
Furthermore, U.S. Pat. No. 7,057,668 has disclosed an image signal processing method for alleviating the CBU phenomenon of the FSC. In a display with red, blue, and green LEDs, or an additional white LED, serving as the backlights, when an image signal is input, it is converted into an YCrCb color system. When a CBU phenomenon of the display content is fairly slight, an image frame is displayed in an FSC manner. When the CBU phenomenon of the display content is rather severe, the backlights are adjusted into all white lights, that is, the red, blue, and green LEDs are all turned on to emit lights, or merely the white LED is turned on to emit lights, thereby suppressing the CBU phenomenon. However, when the backlights are all turned on, color filters are still required for achieving the full-color effects of the image.
Furthermore, Jongseo Lee et al. has published an article entitled “Noble Measurement Method for Color Breakup Artifact in FPDs” in IMID/IDMC'06, in which CIE LUV color coordinates are utilized to analyze the CBU phenomenon, and it is defined that a color difference (ΔE) in the coordinates is a factor for quantification of the CBU. However, in the published document, other novel method for improving the CBU phenomenon is not mentioned.
In terms of alleviating the CBU problem, U.S. Pat. No. 6,911,963 has disclosed an FSC display method for reducing the CBU phenomenon, in which a time sequence of brightness information of an input image information with all the display colors is displayed. In order to display the input image information, that is, synchronously changing the display color and the brightness information, one color image is displayed in at least four sub field intervals in one frame interval, and one picture signal in at least one sub field interval is a non-primary color picture signal, which is generated by at least two primary color signals in the input picture signal carrying primary color signals. The processing manner includes converting the grayscale rgb of the image into a statistical graph of tristimulus values XYZs in a CIE1931XYZ color system, and then converting the statistical graph into corresponding tristimulus values XYZs of backlight colors, thereby determining the color of the additional sub field.
When the above methods are used, the following three conditions must be preset, including:
(1) the CBU easily occurs at a high-frequency portion of a high-brightness (Y value) signal level;
(2) the CBU easily occurs when a frequency of an X value is larger than that of a Z value; and
(3) the CBU easily occurs at a portion with a high Z value, that is, both the X value and the Y value are lower.
Therefore, the color selected from each signal level satisfying the above conditions (1)-(3) is the color of the additional fourth sub field. However, in order to acquire the color of the fourth sub field, the statistics of the image must be analyzed first, which is not only time consuming, but also increases the calculation capacity.
In view of the above problems, the inventor has proposed an adaptive feedback control method of an FSC-LCD, so as to overcome the defects of the prior art.